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From seabirds to Arctic mammals, chimpanzees to orangutans, certain species engage in mouth-to-mouth contact. Currently, scientists propose that Neanderthals also engaged in this behavior – and might even have locked lips with modern humans.

Shared Oral Clues

It is not the first time scientists have proposed ancient relatives and early modern humans were intimately acquainted. In previous studies, scientists have found humans and their Neanderthal relatives shared the same mouth microbe for hundreds of thousands of years after the evolutionary divergence, suggesting they exchanged oral fluids.

"Likely they were engaging in intimate contact," she said, explaining that the idea aligned with research that has found people of non-African ancestry have bits of Neanderthal DNA in their genetic makeup, demonstrating interbreeding was at play.

Romantic Interpretation

"This offers a different perspective on human-Neanderthal relations," the lead researcher commented.

Writing in the publication Evolution and Human Behavior, Brindle and colleagues detail how, to explore the evolutionary origins of intimate contact, they first had to come up with a definition that was not limited to how people kiss.

Defining Kissing

"There have been some efforts to describe a kiss, but it's very much been focused on humans, which means that essentially non-human species do not engage in this. Now we know that they likely engage, it may appear different from what our intimate contact looks like," said the evolutionary biologist.

However, she noted some actions that resembled kissing were distinct activities – such as the chewing and food sharing, or "mouth contact", observed in fish called French grunts.

Consequently the team developed a definition of intimate contact based on friendly interactions involving directed mouth-to-mouth contact with a individual of the same species, with some movement of the mouth but no transfer of food.

Research Methods

The lead researcher explained they focused on accounts of kissing in primates from Africa and Asian regions, including bonobos, apes and orangutans, and employed digital recordings to confirm the observations.

Scientists then integrated this information with information on the genetic connections between extant and extinct species of such animals.

Historical Timeline

Researchers propose the findings indicate intimate contact developed somewhere between 21.5m and 16.9 million years ago in the predecessors of the great primates.

The position of Neanderthals on this family tree suggests it is likely they, too, engaged in a kiss, the researchers conclude. But the activity may not have been limited to their specific group.

"The fact that modern people kiss, the fact that we currently have demonstrated that Neanderthals probably kissed, suggests that the two [species] are probably did engage," the researcher added.

Biological Significance

Although the evolutionary explanation is debated, Brindle explained intimate contact could be used in reproductive situations to possibly increase reproductive success or assist in selecting between partners, while it might help reinforce bonding when practiced in a platonic way.

Another expert in the activities of primates said that as kissing behavior was observed in a broad spectrum of apes it was logical its roots lie deep in our ancient history, and an examination of different forms of intimate behavior among a wider variety of animals might push its beginnings back further still.

"Behaviors that we consider as signatures of our species, like intimate contact, are not unique to us if we look closely at other animals," the expert noted.

Social Aspects

An archaeology expert explained that kissing had a cultural element as it was not common to all societies.

"Nonetheless, as humans we thrive or fail on the strength of our emotional bonds, and ways of promoting trust and intimacy will have been significant for eons," the professor stated. "This could represent an image that appears a bit incongruous to our incorrect assumptions of a supposedly aggressive and ancient history, but actually it should be expected that ancient hominins – and including Neanderthals and our human ancestors together – engaged intimately."